We have developed techniques to fractionate nucleosome oligomers by electrophoresis. Results show that clusters of nucleosomes exist with different DNA repeat sizes, in both calf thymus and cultured mouse cells. The logical explanation is that there are differences in either compositions or arrangements of histones among polynucleosomes. Differences in composition will be explored using (H3)lys, (C14)arg labeled mouse cells. The nucleosome oligomers will first be fractionated by native gel electrophoresis. Next, electrophoresis at right angles to the first dimension will be performed to display histones, using both SDS and acid-urea gels. Bands will be excised, counted, and stoichiometries calculated from known A.A. sequences. Differences in histone arrangements will be explored using reversible chemical cross-linking reagents. Nucleosome oligomers, which have been fractionated by electrophoresis, will be cross-linked in situ and cross-linked products displayed as described above. The protein/DNA ratios in various nucleosome oligomers will be accurately measured by both isotope techniques, and chemical methods after fractionation on Bio-gel A 5 m. A large variety of controls will be performed to test for protein migration both within and between chromatin fibers which might occur during the above experimental approaches.